Eddy Current (EC) instruments are typically used to detect defects, such as cracks and thinning in electrically conductive structures, usually made from metals or to measure material properties such as conductivity and permeability, which in turn are related to hardness and material structure. All of these are observed by monitoring the change in the magnitude and phase of magnetically induced eddy currents. A drive coil that produces an alternating magnetic field is generally provided close to a test piece to induce current in the test piece. The presence of defects disrupts the circulating Eddy Currents. Eddy currents can, be sensed by picking up their magnetic field or by measuring the change of impedance of the drive coil.
There are many arrangements for inducing and sensing eddy currents, but in nearly every case there is a need to measure the phase and amplitude relationship between the sensed and drive signal. This can be done either digitally or by using analogue methods. Often the change in the sensed signal due to a defect is small and electrical noise can mask the measurement. In some cases it is necessary to remove effects on the signal due to other parts of the structure, such as supports. This is often done using multi-frequency drive currents, in which one or more of the frequencies is sensitive to the defect and the others are largely only affected by the support structure.
To achieve good detectivity the instrument has to have both good amplitude and phase discrimination for both single and multi-frequency signals. FIG. 1 shows one way of doing this. A received signal 1 is multiplied by a sine function 2 and a cosine function 3, followed by a low pass filter 4 to obtain the real 5 and imaginary 6 components respectively. These can then either be used directly in a complex impedance plane view of the data or with a little additional computation the phase and amplitude data can be obtained from these numbers.
The multiplication of the signal is readily performed using analog multipliers or can be done digitally using a fast processing device such as an FPGA. Digital methods have the advantage that once the signal has been digitized there are no offset and phase errors. In analog systems the physical components used in the separate real and imaginary signal processing paths will have small differences that result in errors that can be temperature and time dependent.
U.S. Pat. No. 4,207,520 discloses a multiple frequency digital eddy current inspection system. Received eddy current signals from a test coil are filtered to select a desired signal which is changed into a digital signal and passed to a computer for processing and to provide results of the eddy current inspection.
U.S. Pat. No. 6,734,669 discloses digital demodulation of an eddy current signal. Digital sine and cosine functions are generated and multiplied with a digitized received signal. The eddy current signal is isolated by filtering the mixed signal with a low pass filter.